1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus for forming an image on a member to be transferred by an electrophotographic system.
2. Description of the Related Art
An image forming apparatus adopting an electrophotographic system has been conventionally put to practical use. Such an image forming apparatus has been required to achieve a higher quality of an image inclusive of durable stability in recent years. “The durable stability” herein can be rephrased with little variation of a quality of an image from the beginning of the use of the image forming apparatus until expiration of its lifetime. In order to maintain “the durable stability” on an appropriate level, it is important to hold the amount of developer borne on a developer bearing member (hereinafter referred to as toner coat amount) at a proper value from the beginning of the use of the image forming apparatus until the expiration of its lifetime of an apparatus body.
FIG. 11 shows the configuration for restricting the toner coat amount on a developer bearing member 143 in a conventional image forming apparatus. As shown in FIG. 11, the image forming apparatus includes a restricting member 145 for restricting, to a predetermined value, the toner coat amount of developer 147 borne on the developer bearing member 143 (including a magnet roller 146 therein) in contact with the surface. The restricting member 145 abuts against the developer bearing member 143 in a direction counter to the rotational direction (i.e. a Z direction) of the developer bearing member 143, so that a thin layer is formed on a surface of the developer bearing member 143 with the developer in the predetermined toner coat amount. Incidentally, Japanese Patent Application Laid-open No. 2003-66716 discloses a configuration for controlling a toner coat amount using such a restricting member.
However, the image forming apparatus in the prior art has raised the following problems to be solved.
As the image forming apparatus is more frequently used for a longer period of time, the surface roughness of the developer bearing member is reduced due to abrasion, so that the developer is hardly borne. As a consequence, the toner coat amount borne on the developer bearing member is reduced. Since the abutment pressure or fixing position of the restricting member is varied according to the dimensional or fixing tolerance of a part per se, the toner coat amount is fluctuated accordingly. On the other hand, there has been conventionally disclosed a technique for applying a voltage to a restricting member and controlling the voltage, so as to adjust a toner coat amount.
However, although the conventional image forming apparatus can adjust an increase or decrease in toner coat amount, it does not take the toner coat amount before adjustment into consideration, and therefore, it cannot always correct the toner coat amount to a predetermined quantity. Consequently, in the case where the amount of developer borne on a developer bearing member is fluctuated, an image density or the like also is fluctuated, thereby raising a problem to be solved that a good quality of an image cannot be stably achieved.